In the case of lifts, the lift car usually has either a pair of sliding doors which close together, or a single sliding door which closes on a slam post. In either case, the door opening needs to be monitored for obstructions that would prevent door closure. The obstruction may be a human body (or pet), in which case the obstruction detector needs to override the control circuitry for opening and closing the door(s) to prevent injury. The obstruction may also be inanimate, such a patient trolley, hospital bed, shopping trolley, or wheeled pallet, and these objects can become caught in closing doors leading to jamming and damage. In view of these problems, it is common practice to install an obstruction detector in the lift car for monitoring the door opening. The detector is connected to detection circuitry which generates a drive signal in the event of detecting an obstruction, and the drive signal is used for causing door control circuitry to prevent door closure.
Some contemporary lift installations employ a set of infrared diodes for transmitting short wavelength infrared light into a detection zone, and a set of similar diodes for receiving the light either directly or by reflection. With a simple beam interruption system, infrared light transmitter diodes on one side of a door opening transmit light to receiver diodes on the other side of the opening and an obstacle is detected by breaking the beam. These diodes are connected to detection circuitry which responds to beam interruption to generate the drive signal for stopping or reversing door closure. With a reflection system, an obstruction in a detection zone reflects, light onto the receiver diodes and the detection signal generates the drive signal. (This is not a simple case of beam reflection, because the obstruction may have any shape and reflectivity and the transmitter beams are therefore randomly reflected or scattered, and the reflected components which arrive at the receiver diodes have different strengths. This has led to the development of complex circuitry for ensuring reliable operation and for avoiding faulty operation due to stray reflections.)
In some modern beam interruption systems, an array of transmitter diodes are positioned adjacent the closing edge of the lift car door so as to transmit a network of beams of infrared light into the door opening. This provides a so-called "curtain" of light. This "curtain" is present between a pair of sliding doors, when they are open, along the path of sliding door movement. An array of receiver diodes; positioned adjacent the opposite door edge receive the light beams directly from the transmitter diodes and an obstruction is detected when it interrupts the "curtain".
In some modern beam reflection systems, an array of transmitter diodes transmits light into a zone just in front of the door(s) and an array of receiver diodes receive light reflected from, or scattered by the obstruction in this zone. Such systems may be known as infrared "approach" detecting systems.
Both beam interruption and beam reflection systems may be combined for additional safety. In any event, there is the expense of providing the diode arrays and the complex circuitry to enable these arrays to operate effectively.
Neither of these modern systems should be confused with the now obsolete proximity detectors which included capacitance detectors mounted on the edges of sliding doors. These were subject to errors due to the difficulty of reliably balancing capacitances and maintaining the balance. This balancing needed to be carried out initially by skilled operatives at each particular lift location so that the detection circuitry responded only to an obstruction. Moreover, complex detection circuitry was needed for maintaining the balance.
Regardless of the system used, problems can be caused due to the doors closing. Where diodes are set into door edges, the strength of the detection signal Will increase as the doors close, due to the diminishing space between the transmitters and receivers, and hence some form of gain control is necessary to prevent faulty operation. With capacitive sensors, the balance will change as the doors close, due to the capacitances of the doors and/or slam post and compensating circuitry is required to counteract this. Such measures also clearly contribute complexity and expense.